The lower audibility limit is defined as SPL of 0 dB, but the upper limit is not as clearly defined. While 1 atm (194 dB Peak or 191 dB SPL) is the largest variation in pressure that an undistorted sound wave can have in the Earth’s atmosphere, larger sound waves can be present in other atmospheres or other media as underwater or through the Earth.
Contour with equal volume
The ears detect changes in sound pressure. Human hearing does not have a flat spectral sensitivity (frequency response) relative to frequency versus amplitude. Humans do not perceive low and high frequency sounds and perceive sounds between 3,000 and 4,000 Hz, because the frequency response of human hearing changes with amplitude, three weightings have been established to measure sound pressure: A, B and C. A weighting applies to sound pressure levels up to 55 dB, weighting B applies to sound pressure levels between 55 dB and 85 dB and weighting C is used to measure sound pressure levels above 85 dB.
To distinguish the different sound measurements, a suffix is used: the A-weighted sound pressure level is written as dBA or LA. The weighted sound pressure level B is written both as dBB and as LB, and the weighted sound pressure level C is written as dBC or LC.
The unweighted sound pressure level is called the “linear sound pressure level” and is often written as dBL or simply L. Some sound measurement instruments use the letter “Z” as an indication of linear SPL.